Detecting signals from flexible media such as flexible disk files and magnetic tape presents unique detection requirements since in such systems the head is generally not in contact with the recording medium and data is stored in run length limited codes where a bandwidth limited version of the derivative of the write wave form is provided upon reading. In that environment, the detection of the readback system must be amplitude sensitive since the raw data provided by the magnetic transducer has discrete positive, zero and negative levels.
Moreover, since magnetic coatings upon flexible media are more susceptible to dropouts than rigid media, relatively large variations of signal amplitude are encountered. Additional problems are encountered since data recorded in a run length limited code usually includes overwriting of data on the track being detected. This introduces noise signals which the detector must be able to discriminate.
These problems have been overcome in a system described in U.S. Pat. No. 4,346,411 titled Amplitude Sensitive Three-Level Detector for Derivative Readback Channel of Magnetic Storage Device. The aforementioned patent describes a system which provides improved detection capability with respect to signals stored upon flexible magnetic media. The patented system generally includes a magnetic transducer for providing an analog signal that represents stored binary data, comparator means for detecting positive and negative peaks of the analog signal, and voltage threshold means for providing a reference threshold voltage to the comparators which is a precise percentage of the voltage peaks of the analog signal.
It has been found that in such a system, a need still exists for providing a magnetic recording detection pointer that gives a reliable indication of marginal detection environments in which a high probabilitv of detection error exists. This need especially exists in high data rate channel processing utilizing a high density analog sampling detector such as that described in the aforementioned patent.
The use of magnetic recording detection pointers in conjunction with magnetic recording channels is known. Thus, error correction is typically used in conjunction with data detection systems to provide enhanced data reliability. Typically, in a multitrack environment, the correction capability of an implemented error correction algorithm will exceed the detection capability of the system. This means that beyond some maximum number of tracks in error in the system, the error correction algorithm cannot by itself correctly identify additional error locations. Consequently, other indicators of likely error occurrence, i.e. pointers, are used to extend the system's capability.
Pointers historically used with analog sampling detection systems employed in conjunction with magnetic recording schemes have typically been of two types, i.e. amplitude pointers and phase pointers.
An amplitude pointer generally provides a means of determining when the envelope in the analog wave form has decreased below a minimum fixed level that represents a valid data transition or pulse. A phase pointer generally is utilized to indicate that a shift in signal phase has occurred within a given data cell or envelope beyond a fixed maximum interval. This implies a false data signal or imminent detection failure.
It has been recognized that the use of either amplitude or phase detection pointers in conjunction with a system such as that described in the aforementioned U.S. Pat. No. 4,346,411 would be ineffective. Thus, the patented system describes a technique for detecting positive and negative peaks that represent data using comparator means and voltage threshold means that provide to the comparator a reference threshold voltage which is a precise percentage of the voltage peaks of the analog signal. The existence of a valid peak is taken to be a signal that has an amplitude in excess of the variable threshold at a point which is taken to be the center of the pulse based upon a clocking signal derived from the peak of the differential signal. With this type of detection scheme, neither an amplitude pointer nor a phase pointer would provide reliable information beyond that already available from the detection circuit. Accordingly, a need exists for a new type of detection pointer which can be utilized in conjunction with the patented system for providing more reliable overall performance.